EVALUATION ON THE PERFORMANCES OF A MICRO ...umpir.ump.edu.my/8675/1/cd8211.pdfmesin menunjukkan...
Transcript of EVALUATION ON THE PERFORMANCES OF A MICRO ...umpir.ump.edu.my/8675/1/cd8211.pdfmesin menunjukkan...
EVALUATION ON THE PERFORMANCES OF A MICRO GAS TURBINE, GASENGINE-COGENERATION SYSTEM IN SEWAGE TREATMENT PLANT
MOHAMAD ARIF BIN ROSLAN
Report submitted in partial fulfillment of the requirementsFor the award of
Bachelor of Mechanical Engineering with Automotive Engineering
Faculty of Mechanical EngineeringUNIVERSITY MALAYSIA PAHANG
JUNE 2013
vii
ABSTRACT
This paper provides an overview on the characterization of the efficiency and
emissions from small-scale cogeneration systems, with specific reference to the prime
movers technologies nowadays most available in the market, namely, micro gas turbine
and gas engines. The corresponding environmental impacts are evaluated by using the
emission balance approach. A numerical case study with two representative machines
highlights their different emission characteristics, also considering the partial-load
emission performance. Power produced from micro gas turbine and gas engine will
supply to power demand while heat from both of cogeneration as stated above will
undergo heat exchanger to produce heat exhaust heat recovery, Qehr. Emission and
efficiency data are obtained from the journal. It calculated by using formula to calculate
efficiency of the systems. In order to get the best configuration of cogeneration in
sewage treatment plants in term of efficiency and emission there is one system. By
using the micro gas turbine as a base that run on full load while gas engine run on
partial load, two emission reduced to 20% from its original volume of 1st month, they
are nitrogen oxide and carbon monoxide. Nitrogen oxide emission from 48x103 kg
reduced to 38x103 kg and Carbon monoxide from 1.5x107 kg to 11964340 kg. For
efficiency it increases by 2.33% from previous efficiency.
viii
ABSTRAK
Kajian ini memberikan gambaran mengenai pencirian kecekapan dan pelepasan
gas dari sistem penjanaan kecil-kecilan, dengan merujuk khusus kepada penggerak
utama teknologi pada masa kini yang paling boleh didapati di pasaran, iaitu turbin gas
mikro dan enjin gas. Kesan alam sekitar juga dinilai dengan menggunakan pendekatan
keseimbangan pembebasan gas. Satu kajian kes berangka dengan menggunakan dua
mesin menunjukkan ciri-ciri pembebasan gas yang berbeza, juga mengambil kira
prestasi pelepasan separa-beban. Kuasa yang dihasilkan daripada gas turbin mikro dan
enjin gas akan dibekalkan kepada permintaan kuasa manakala haba dari kedua-dua
penjanaan yang dinyatakan di atas akan menjalani penukar haba untuk menghasilkan
haba ekzos pemulihan haba, Qehr. Pembebasan gas dan kecekapan data diperolehi
daripada jurnal. Ia dikira dengan menggunakan formula untuk mengira kecekapan
sistem. Dalam usaha untuk mendapatkan konfigurasi terbaik penjanaan dalam loji
rawatan kumbahan dari segi kecekapan dan pembebasan gas terdapat satu sistem.
Dengan menggunakan turbin gas mikro sebagai asas yang berjalan pada beban penuh
manakala enjin gas dijalankan pada beban separa, dua pelepasan dikurangkan kepada
20% dari jumlah asal pada bulan pertama, Ia adalah nitrogen oksida dan karbon
monoksida. Nitrogen oksida membebaskan 48x103 kg dan dikurangkan kepada 38x103
kg dan karbon monoksida daripada 1.5x107 kg kepada 11964340 kg. Untuk kecekapan
ia meningkat sebanyak 2.33% daripada kecekapan sebelumnya.
ix
TABLE OF CONTENTS
Page
EXAMINER APPROVAL DOCUMENTS II
SUPERVISOR’S DECLARATION III
STUDENT’S DECLARATION IV
DEDICATION V
ACKNOWLEDGEMENTS VI
ABSTRACT VII
ABSTRAK VIII
TABLE OF CONTENTS IX
LIST OF TABLES XII
LIST OF FIGURES XIII
NOMENCLATURE XIV
CHAPTER 1 INTRODUCTION
1.1 Project Background 1
1.1.1 Sewage treatment plant 1
1.1.2 Biogas 4
1.1.3 Cogeneration 4
1.2 Problem Statement 5
1.3 Objective of study 5
1.4 Scope of Project 6
CHAPTER 2 LITERATURE REVIEW
2.1 World Energy Problems 7
2.2 Process of Sewage Treatment Plants 9
2.3 Uses of Biogas 12
2.4 Concept of Cogeneration (CHP) 13
2.5 Micro Gas Turbine Cogeneration Systems 15
x
2.6 Gas Engine Cogeneration Systems 16
CHAPTER 3 METHODOLOGY
3.1 Introduction 18
3.2 System of MGT-GE plants 20
3.3 Prime mover Specification 20
3.4 Exhaust Design 21
3.5 Emission of CO, NOx from GE-MGT 22
3.5.1 Emission from MGT 22
3.5.2 Emission from GE 24
3.6 Operation method 25
3.7 Energy balance calculation 26
CHAPTER 4 RESULT AND DISCUSSION
4.1 Introduction 28
4.2 Optimum Condition 28
4.3 Energy balance 29
4.3.1 Heat 29
4.3.2 Electrical 30
4.4 Emission 31
4.4.1 NOx emission 31
4.4.2 CO emission 32
4.4.3 Emission analysis 33
4.5 Efficiency 34
4.5.1 Power efficiency 34
CHAPTER 5 CONCLUSION
5.1 Conclusion 36
5.2 Recommendation 37
xi
REFERENCES 38
APPENDIX
A1 Gantt Chart FYP1 40
A2 Gantt Chart FYP2 41
B1 Raw Data of Experiment 42
xii
LIST OF TABLE
Table No. Title Page
3.1 MGT CGS and GE CGS specification 22
3.2 Emission of MGT CGS 23
3.3 Emission of GE CGS 24
4.1 Load condition for MGT CGS 34
xiii
LIST OF FIGURE
Figure Title Page
1.1 NOx emission versus electrical power 2
1.2 Torque Curve for Intake and Exhaust Tuning 5
2.1 CHP versus Separate Heat and Power(SHP) Production 14
2.2 Micro Gas Turbine Cogeneration 16
2.3 Gas Engine Cogeneration 17
3.1 Flow Chart Methodology 19
3.2 MGT-GE CGS plant System 20
3.3 Parameter of the model sewage treatment plant 21
3.4(a) NOx emission versus electrical power 23
3.4(b) CO emission versus electrical power 24
3.5(a) NOx emission versus electrical power 25
3.5(b) CO emission versus electrical power 25
3.6 Biogas produced, Heat demand and Power demand
versus average month 26
4.1 Optimum condition for MGT, GE CGS 29
4.2 Heat demand, Qehr GE and Qehr MGT-GE versus
monthly (2004-2007) 30
4.3 Power demand, Pe-MGT and Pe-MGT-GE versus
monthly (2004-2007) 31
4.4 NOx emission versus average month 32
4.5 CO emission versus average month 33
4.6 NOx emission for MGT-CGS for Month 6 and 7 33
4.7 Power efficiency versus average month 35
xiv
NOMENCLATURE
ci Cold fluid (water) inlet
conv. Conventional system
co Cold fluid (water) outlet
dig Digestion
ehr Exhaust heat recovery
EPDR Electrical power demand reduction
exe Exhaust
FL Full load
g.c. Gas compressor
h.d. Heat demand
ho Hot fluid outlet
MGT Micro gas turbine
GE Gas engine
PL Partial load
r.b. Remaining biogas
r.h.d. Remaining heat demand
CGS Cogeneration
M Mass or volume flow rate, kg/s or m/s
Pe Electrical power, kW
PL Partial load ratio, e
Q Heat energy, kW
t Temperature,
η Efficiency
y Emission
x MGT or GE unit
CHAPTER 1
INTRODUCTION
1.1 PROJECT BACKGROUND
The world is now facing two energy-related threats, the treat of rapid depletion
of fossil fuel and the treat of environmental disruption (International agency energy,
2006). Thus, the utilization of renewable energy and application of highly efficiency
technology are important for the development of a sustainable energy system (Sovacool
BK, 2009). Interest has recently been shown in the utilization of biomass by a
cogeneration system (CGS) and its applications have been increasing.
1.1.1 Sewage treatment plant
Sewage can be treated close to where it is created, a centralized system (in septic
tanks, bio filters or aerobic treatment systems), or be collected and transported by a
network of pipes and pump stations to a municipal treatment plant, a centralized system.
Sewage collection and treatment is typically subject to local, state and federal
regulations and standards. Industrial sources of sewage often require specialized
treatment processes. Sewage treatment generally involves three stages, called primary,
secondary and tertiary treatment.
Primary treatment consists of temporarily holding the sewage in a quiescent
basin where heavy solids can settle to the bottom while oil, grease and lighter solids
float to the surface. The settled and floating materials are removed and the remaining
liquid may be discharged or subjected to secondary treatment.
2
Secondary treatment removes dissolved and suspended biological matter.
Secondary treatment is typically performed by indigenous, water-borne micro-
organisms in a managed habitat. Secondary treatment may require a separation process
to remove the micro-organisms from the treated water prior to discharge or tertiary
treatment.
Tertiary treatment is sometimes defined as anything more than primary and
secondary treatment in order to allow rejection into a highly sensitive or fragile
ecosystem . Treated water is sometimes disinfected chemically or physically (for
example, by lagoons and microfiltration) prior to discharge into a stream, river, bay,
lagoon or wetland, or it can be used for the irrigation of a golf course, green way or
park. If it is sufficiently clean, it can also be used for groundwater recharge or
agricultural purposes.
Figure 1.1 : Sewage Treatment Plant Process.
Source : (Renewable energy,Sewage Treatment System, 2011)
The sludges accumulated in a wastewater treatment process must be treated and
disposed of in a safe and effective manner. The purpose of digestion is to reduce the
amount of organic matter and the number of disease-causing microorganisms present in
3
the solids. The most common treatment options include anaerobic digestion, aerobic
digestion, and composting. Incineration is also used, albeit to a much lesser
degree.(EPA, Washington, DC 2004). Sludge treatment depends on the amount of solids
generated and other site-specific conditions. Composting is most often applied to small-
scale plants with aerobic digestion for mid sized operations, and anaerobic digestion for
the larger-scale operations. The sludge is sometimes passed through a so-called pre-
thickener which de-waters the sludge. Types of pre-thickeners include centrifugal
sludge thickeners (IPEC Consultant, Ltd , Burnaby, BC, Canada 2012 ) rotary drum
sludge thickeners and belt filter presses.
Anaerobic digestion is a bacterial process that is carried out in the absence of
oxygen. The process can either be thermophilic digestion, in which sludge is fermented
in tanks at a temperature of 55 °C, or mesophilic, at a temperature of around 36 °C.
Though allowing shorter retention time (and thus smaller tanks), thermophilic digestion
is more expensive in terms of energy consumption for heating the sludge.
Anaerobic digestion is the most common (mesophilic) treatment of domestic
sewage in septic tanks, which normally retain the sewage from one day to two days,
reducing the biochemical oxygen demand (BOD) by about 35 to 40 percent. This
reduction can be increased with a combination of anaerobic and aerobic treatment by
installing Aerobic Treatment Units (ATUs) in the septic tank. Mesophilic anaerobic
digestion (MAD) is also a common method for treating sludge produced at sewage
treatment plants. The sludge is fed into large tanks and held for a minimum of 12 days
to allow the digestion process to perform the four stages necessary to digest the sludge.
These are hydrolysis, acidogenesis, acetogenesis and methanogenesis. In this process
the complex proteins and sugars are broken down to form more simple compounds such
as water, carbon dioxide and methane.
One major feature of anaerobic digestion is the production of biogas (with the
most useful component being methane), which can be used in generators for electricity
production and/or in boilers for heating purposes. Many larger sites utilize the biogas
for combined heat and power, using the cooling water from the generators to maintain
the temperature of the digestion plant at the required 35 ± 3 °C.
4
1.1.2 Biogas
Biogas typically refers to a gas produced by the breakdown of organic matter in
the absence of oxygen. It is a renewable energy source, like solar and wind energy.
Furthermore, biogas can be produced from regionally available raw materials and
recycled waste and is environmentally friendly. Biogas is produced by the anaerobic
digestion with anaerobic bacteria or fermentation of biodegradable materials such as
manure, sewage, municipal waste, green waste, plant material, and crops. Biogas
comprises primarily methane (CH4) and carbon dioxide (CO2) and may have small
amounts of hydrogen sulphide (H2S), moisture and siloxanes. The gases methane,
hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen.
This energy release allows biogas to be used as a fuel. Biogas can be used as a
fuel in any country for any heating purpose, such as cooking. It can also be used in
anaerobic digesters where it is typically used in a gas engine to convert the energy in the
gas into electricity and heat
Biogas produced from sewage treatment plant is in form of methane. To get this
biogas it needs constant temperature within range 39˚ - 55˚ C.
1.1.3 Cogeneration (CGS)
Figure 1.2 shows the efficiency advantage of CGS compared with conventional
central station power generation and onsite boilers. When considering both thermal and
electrical processes together, CGS typically requires only ¾ the primary energy separate
heat and power systems require. CGS systems utilize less fuel than separate heat and
power generation, resulting for same level of output, resulting in fewer emissions. Many
of the benefits of CGS stem from the relatively high efficiency of CGS systems
compared to other systems. Because CGS systems simultaneously produce electricity
and useful thermal energy, CGS efficiency is measured and expressed in a number of
different ways.
5
Figure 1.2 : Cogeneration (CGS) or CHP versus Separate Heat and Power
1.2 PROBLEM STATEMENT
MGT-CGS when runs on partial load condition it has high emission and also
low in term of performances efficiency. But for GE-CGS, it emits low emission of NOx
and CO under partial load condition as well as has high on efficiency of performances.
Therefore for the problematic point for this research is how to combine MGT-CGS and
GE-CGS in sewage treatment plant by considering the performances of energetic and
environmental.
1.3 OBJECTIVE OF STUDY
i. To optimize performances of a Micro Gas Turbine, Gas Engine Cogeneration
(MGT-CGS, GE-CGS) in sewage treatment plants.
ii. To clarify best configuration of cogeneration in sewage treatment plants.
6
1.4 SCOPE OF PROJECT
i. For MGT and GE by using small scale which 300kW and below
ii. Limitation using small scale on performances of energetic and
environmental.
iii. For sewage under small scale use approximation on 150 000 people.
CHAPTER 2
LITERATURE REVIEW
2.1 WORLD ENERGY PROBLEMS
The world is now facing two energy-related threats, the treat of rapid depletion
of fossil fuel and the treat of environmental disruption (International Energy
Agency,2006). Alternative energy offers a set of renewable, sustainable and clean
sources of energy that offer a new way of generating the power and fuel that peoples
need as the more traditional forms of energy, such as fossil fuels, become depleted.
Alternative energy comes in a number of different forms, such as solar, wind, hydro-
electric, biomass and biogas power. These sources of energy are all renewable, which
means that no matter how much they are used, there will always be more energy
available to use. This makes them a reliable source of energy for the future and an
alternative to the non-renewable energy sources that will one day run out.
Alternative energy also offers some other important advantages over the use of
fossil fuels. Using fossil fuels to produce energy also produces pollution, which can
result in serious health and environmental problems. Finding cleaner sources of energy
can help to prevent these problems, as well as ensuring that there will be a reliable and
renewable source of energy available in the future. Fossil fuels are a non-renewable
source of energy. This means that as they are used up, they are not being replaced. Once
gone, they are gone forever. At some point in the future, it will not be possible to
continue relying on fossil fuels as a source of energy, so an alternative energy supply
will need to be found.
Alternative energy provides an important alternative to fossil fuels. These
alternatives are likely to become increasingly important as concern about the pollution
8
generated by burning fossil fuels grows, but one of the main reasons why it is important
to learn more about alternative energy is that fossil fuels are a dead end. As they are
depleted, people will need to make increasing use of other sources of energy in order to
continue enjoying the same lifestyle that we do today.
There are many views out there in regard to fossil fuels. Many people believe
that they are the only way for our world to survive, while others believe that finding
alternative fuel sources should be something that is given one hundred percent of our
attention. Which is right? Fossil fuels are a dead end that is approaching must faster
than most had anticipated. However, many people are still under the assumption that
fossil fuels are not coming to an end, but they are wrong.
Fossil fuels are slowly becoming obsolete. Not in that they are no longer being
used. But, rather that they are getting to the point that they are running out. Fossil fuels
are not a replenishing source of power and right now drilling companies are trying to
find more in order to meet the needs of the world. When these sources run out, there
will be nothing that is left, which is why alternative fuel sources need to be found that
are in abundance and use items that never run out such as the use of solar power that
relies on the Sun, or the use of hydroelectricity that rely on water to power homes and
businesses, as well as vehicles.
In addition, the cost of fossil fuels is outrageous. Most people are paying over
three dollars for one gallon of gasoline to power their vehicles, however driving solar
powered cars would be much less expensive. And since it is something that people
cannot do without, they simply have to shoulder the cost and find ways in which they
can budget the rest of their lives. The cost of these fuels will only continue to rise as the
demand increases and the supply decreases. It is the simply law of supply and demand.
When the supply is completely, then there will be no where for people to turn in order
to get power.
Fossil fuels are also responsible for the dangerous amounts of chemicals in the
atmosphere that are leading to the increased effects of the greenhouse effect. These
effects are starting to be felt in places throughout the world, such as Alaska where ice
caps are melting. Hundreds of years ago, this was never given enough consideration,
9
however, the effect that it is having on the animals in the environment are going to be
those that the world cannot reverse once they happen.
The only hope that the world has in order to survive in the coming years, is to
find those alternative fuels that are not going to affect the environment in such a harsh
way, as well as help those people in the world be able to afford their way of life. If
nothing is done, and no research is put into finding these alternative fuels then the future
is uncertain. For those that believe our reliance upon fossil fuels as sources of power is
not going to be a story of devastation, then they have not considered the facts and the
problems that are occurring because fossil fuels are being used. Fossil fuels are
essentially a dead end for the world.
2.2 PROCESS OF SEWAGE TREATMENT PLANTS
Sewage treatment is the process of removing contaminants from wastewater and
household sewage, both runoff (effluents), domestic, commercial and institutional. It
includes physical, chemical, and biological processes to remove physical, chemical and
biological contaminants. Its objective is to produce an environmentally safe fluid waste
stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or
reuse (usually as farm fertilizer). Using advanced technology it is now possible to re-use
sewage effluent for drinking water, although Singapore is the only country to implement
such technology on a production scale in its production of NEWater.(Singapore
National Water Agency, 2011)
Sewage is generated by residential, institutional, commercial and industrial
establishments. It includes household waste liquid from toilets, baths, showers, kitchens,
sinks and so forth that is disposed of via sewers. In many areas, sewage also includes
liquid waste from industry and commerce. The separation and draining of household
waste into greywater and blackwater is becoming more common in the developed
world, with greywater being permitted to be used for watering plants or recycled for
flushing toilets.
Sewage may include stormwater runoff. Sewerage systems capable of handling
storm water are known as combined sewer systems. This design was common when
10
urban sewerage systems were first developed, in the late 19th and early 20th centuries
(Metcalf et al.,1972). Combined sewers require much larger and more expensive
treatment facilities than sanitary sewers. Heavy volumes of storm runoff may
overwhelm the sewage treatment system, causing a spill or overflow. Sanitary sewers
are typically much smaller than combined sewers, and they are not designed to transport
stormwater. Backups of raw sewage can occur if excessive infiltration/inflow (dilution
by stormwater and/or groundwater) is allowed into a sanitary sewer system.
Communities that have urbanized in the mid-20th century or later generally have built
separate systems for sewage (sanitary sewers) and stormwater, because precipitation
causes widely varying flows, reducing sewage treatment plant efficiency (Burrian et al.,
1999).
As rainfall travels over roofs and the ground, it may pick up various
contaminants including soil particles and other sediment, heavy metals, organic
compounds, animal waste, and oil and grease. (Lewis Publisher, 2001).Some
jurisdictions require stormwater to receive some level of treatment before being
discharged directly into waterways. Examples of treatment processes used for
stormwater include retention basins, wetlands, buried vaults with various kinds of media
filters, and vortex separators.(to remove coarse solids).
Sewage can be treated close to where it is created, a decentralised system (in
septic tanks, biofilters or aerobic treatment systems), or be collected and transported by
a network of pipes and pump stations to a municipal treatment plant, a centralised
system (see sewerage and pipes and infrastructure). Sewage collection and treatment is
typically subject to local, state and federal regulations and standards. Industrial sources
of sewage often require specialized treatment processes. Tertiary treatment is sometimes
defined as anything more than primary and secondary treatment in order to allow
rejection into a highly sensitive or fragile ecosystem . Treated water is sometimes
disinfected chemically or physically (for example, by lagoons and microfiltration) prior
to discharge into a stream, river, bay, lagoon or wetland, or it can be used for the
irrigation of a golf course, green way or park. If it is sufficiently clean, it can also be
used for groundwater recharge or agricultural purposes.
11
The sludges accumulated in a wastewater treatment process must be treated and
disposed of in a safe and effective manner. The purpose of digestion is to reduce the
amount of organic matter and the number of disease-causing microorganisms present in
the solids. The most common treatment options include anaerobic digestion, aerobic
digestion, and composting. Incineration is also used, albeit to a much lesser
degree.(EPA, Washington, DC 2004). Sludge treatment depends on the amount of solids
generated and other site-specific conditions. Composting is most often applied to small-
scale plants with aerobic digestion for mid sized operations, and anaerobic digestion for
the larger-scale operations. The sludge is sometimes passed through a so-called pre-
thickener which de-waters the sludge. Types of pre-thickeners include centrifugal
sludge thickeners (IPEC Consultant, Ltd , Burnaby, BC, Canada 2012 ) rotary drum
sludge thickeners and belt filter presses. Anaerobic digestion is a bacterial process that
is carried out in the absence of oxygen. The process can either be thermophilic
digestion, in which sludge is fermented in tanks at a temperature of 55 °C, or
mesophilic, at a temperature of around 36 °C. Though allowing shorter retention time
(and thus smaller tanks), thermophilic digestion is more expensive in terms of energy
consumption for heating the sludge.
Anaerobic digestion is the most common (mesophilic) treatment of domestic
sewage in septic tanks, which normally retain the sewage from one day to two days,
reducing the biochemical oxygen demand (BOD) by about 35 to 40 percent. This
reduction can be increased with a combination of anaerobic and aerobic treatment by
installing Aerobic Treatment Units (ATUs) in the septic tank. Mesophilic anaerobic
digestion (MAD) is also a common method for treating sludge produced at sewage
treatment plants. The sludge is fed into large tanks and held for a minimum of 12 days
to allow the digestion process to perform the four stages necessary to digest the sludge.
These are hydrolysis, acidogenesis, acetogenesis and methanogenesis. In this process
the complex proteins and sugars are broken down to form more simple compounds such
as water, carbon dioxide and methane.
One major feature of anaerobic digestion is the production of biogas (with the
most useful component being methane), which can be used in generators for electricity
production and/or in boilers for heating purposes. Many larger sites utilize the biogas
12
for combined heat and power, using the cooling water from the generators to maintain
the temperature of the digestion plant at the required 35 ± 3 °C.
2.3 USES OF BIOGAS
Everyone is looking for the next big thing that is going to power the world,
while also lowering the amount of toxins that are released into the environment, and
being a safer alternative. One of the alternatives that are getting some huge publicity is
the power of biogas. Biogas is made from waste that piles up in landfills as well as other
waste. This seems a bit outrageous and disgusting, however it is not by any means a
new discovery. This was one fuel source that has been around for while, however it got
put onto the back burner with the development of fossil fuels and other power options
came onto the scene.
The materials that are used to make biogas produce the gas when the materials
are lacking of oxygen. The most common way in which it is made through taking these
sources to the lab and then locking them into an air tight box, where they can get no
oxygen. When the levels of oxygen begin to leave, the gas is produced, which is then
harnessed to be put to use. However, biogas is also naturally produced in landfills.
Those things that are on the bottom of the landfill never see the light of day, nor do that
get any oxygen, thus they are releasing this gas into the air. The key is to catch the gas
so that it can be used to power the world. There has been the idea proposed that if the
government were to install pipes underneath the landfills that they could catch this
biogas that is being produced naturally, thus eliminating the need to have to make the
gas in the plant.
The process sounds fairly easy since it is produced naturally in the environment
where there is waste present. However, the key is getting the biogas to power plants.
This is where people are coming to a dead end. Transporting this gas is not efficient as
it allows a lot to escape, thus those places that were to rely on biogas really need to be
located within the vicinity of where the gas is being produced. That way the biogas can
be transported to them through the use of pipes like the ones that catch the gas. There is
also the problem of having enough biogas produced in order to power the world.
Though there are tons of landfills with hundreds of tons of garbage, scientists say that
13
this is not enough to produce enough to power the world as they are experiencing the
power right now. Other researchers point out that biogas would have to be the
secondary choice because of this aspect, which means that it will not totally relieve the
reliability that people are having on other forms of power. So many people are viewing
this as a dead end.
There still needs to be much testing in order to develop the process further to see
if biogas is a power option for the future. However, there are those that are using this
technology right now and finding success with it.
2.4 CONCEPT OF COGENERATION ( CGS )
Fueled by electric industry deregulation, environmental concerns, unease over
energy security, and a host of other factors, interest in cogeneration (CGS) technologies
has been growing among energy customers, regulators, legislators, and developers.
CGS is a specific form of distributed generation (DG), which refers to the strategic
placement of electric power generating units at or near customer facilities to supply on-
site energy needs. CGS enhances the advantages of DG by the simultaneous production
of useful thermal and power output, thereby increasing the overall efficiency. CGS
offers energy and environmental benefits over electric-only and thermal-only systems in
both central and distributed power generation applications. CGS systems have the
potential for a wide range of applications and the higher efficiencies result in lower
emissions than separate heat and power generation system. The advantages of CGS
broadly include the simultaneous production of useful thermal and electrical energy in
CGS systems lead to increased fuel efficiency. CGS units can be strategically located at
the point of energy use. Such on site generation avoids the transmission and distribution
losses associated with electricity purchased via the grid from central stations. CGS is
versatile and can be coupled with existing and planned technologies for many different
applications in the industrial, commercial, and residential sectors.
CGS is the sequential or simultaneous generation of multiple forms of useful
energy (usually mechanical and thermal) in a single, integrated system. CGS systems
consist of a number of individual components—prime mover (heat engine), generator,
14
heat recovery, and electrical interconnection—configured into an integrated whole. The
type of equipment that drives the overall system (i.e., the prime mover) typically
identifies the CGS system. Prime movers for CGS systems include reciprocating
engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells.
These prime movers are capable of burning a variety of fuels, including natural gas,
coal, oil, and alternative fuels to produce shaft power or mechanical energy. Although
mechanical energy from the prime mover is most often used to drive a generator to
produce electricity, it can also be used to drive rotating equipment such as compressors,
pumps, and fans. Thermal energy from the system can be used in direct process
applications or indirectly to produce steam, hot water, hot air for drying, or chilled
water for process cooling.
Figure 1 shows the efficiency advantage of CGS compared with conventional
central station power generation and on-site boilers. When considering both thermal
and electrical processes together, CGS typically requires only ¾ the primary energy
separate heat and power systems require. This reduced primary fuel consumption is key
to the environmental benefits of CGS, since burning the same fuel more efficiently
means fewer emissions for the same level of output.
Figure 2.1: Cogeneration(CGS) or CHP versus Separate Heat and Power (SHP)
Production
Source : Tina Kaarsberg and Joseph Roop, “Combined Heat and Power: How Much
Carbon and Energy Can It Save for Manufacturers?”
15
Many of the benefits of CGS stem from the relatively high efficiency of CGS
systems compared to other systems. Because CGS systems simultaneously produce
electricity and useful thermal energy, CGS efficiency is measured and expressed in a
number of different ways. Sewage treatment generally involves three stages, called
primary, secondary and tertiary treatment. Primary treatment consists of temporarily
holding the sewage in a quiescent basin where heavy solids can settle to the bottom
while oil, grease and lighter solids float to the surface. The settled and floating materials
are removed and the remaining liquid may be discharged or subjected to secondary
treatment. Secondary treatment removes dissolved and suspended biological matter.
Secondary treatment is typically performed by indigenous, water-borne micro-
organisms in a managed habitat. Secondary treatment may require a separation process
to remove the micro-organisms from the treated water prior to discharge or tertiary
treatment.
2.5 MICRO GAS TURBINE COGENERATION SYSTEM
MGTs are gas turbines that operate on the basis of the Brayton cycle, and
recently developed MGTs are equipped with a recuperator and therefore they have
higher efficiency. The biogas-fuelled MGTCGS was mainly composed of the MGT, and
an exhaust heat exchanger (EHE), and the MGT was a regenerative single shaft MGT.
Atmospheric air enters the MGT passing through a generator, a compressor, a
recuperator, a combustion chamber, a turbine and then the recuperator. MGT-CGSs
with electrical power outputs of 30 kW (MGT30) were investigated in this study, and
their design parameters under ISO conditions . A heat exchanger with temperature
exchange efficiency 3 of 80% was assumed to be used.
The recovered exhaust heat Q from the exhaust heat exchanger was analyzed by
the NTUe represent the inlet and the outlet temperatures of the cold(water) side of the
exhaust heat exchanger, respectively, and t exe and t represent the inlet and outlet
temperatures of the hot(exhaust gas) side of the exhaust heat exchanger, respectively.
Inlet temperature on the cold side of the exhaust heat exchanger was assumed to be
equal to 80 ho C. Unlike other prime movers, performance of MGTs is strongly affected
by the inlet ambient temperature.